This invention relates to solvent-treated polymer powder. In particular it relates to novel polyacrylonitrile powders combined with solvent and to a method for preparing free flowing particles containing a large amount of solvent.
Homopolymers and interpolymers of acrylonitrile are useful in a variety of film and fiber applications. In many of these manufacturing processes, the polyacrylonitrile (PAN) resin is dissolved as a dry powder in a suitable solvent, such as dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO), and cast or extruded into the desired shape at elevated temperature and pressure. Of particular interest in recent years are gas impermeable films made from clear solutions of PAN and DMSO, such as disclosed in U.S. Pat. No. 4,066,731. In such applications, homogeneous PAN solutions are necessary to obtaining good quality product. Undissolved solids or gases cause discontinuities which can affect appearance, film integrity and performance deficiencies.
When forming extruded or cast shapes continuously from PAN solutions, especially at moderate temperature and pressure, it is desirable to use a concentrated solution containing about 30 to 50 percent by weight of PAN. While such concentrations are attainable by prior methods of dissolution, excessively long solution times or very high temperatures are often required to obtain the necessary degree of homogeneity. Large amounts of solvent are required in order to obtain complete dissolution quickly. While the excess solvent may be removed by evaporation, this contributes to the higher cost of processing. Attempts to dissolve dry PAN powder with hot organic solvents at high concentrations by conventional methods have not been successful due to the tendency of the particles to adhere to one another and form a sticky mass of agglomerates. Partially solvated polymeric mixtures have a high viscosity and are difficult to dissolve because the fresh solvent cannot effectively penetrate the coalesced mass. In U.S. Pat. No. 4,028,302 a process is disclosed for making clear acrylonitrile polymer solutions from powder and high-boiling solvents, such as dimethyl formamide or dimethyl sulfoxide. The tendency of polymeric particles to form agglomerates during ordinary solution methods, coalescing into dough balls of partially solvated masses, is overcome by the use of a particular twin-screw machine. The process involves feeding polyacrylonitrile powder into a cool zone of the machine, and injecting organic solvent subsequently in the cool zone at a predetermined rate to form a relatively concentrated mixture in slurry, which is then heated while intermittently squeezing the slurry transversely to the screw axes. The clear, bubble-free solution is discharged at about 110.degree. 150.degree. C.
Prior attempts to dissolve acrylonitrile homopolymers and copolymers have generally required much larger amounts of organic solvent to be employed in the dissolution step than is required in extruding the product. For instance, a casting dope can be made by dissolving 18% PAN homopolymer in DMSO at 50.degree. C. and evaporating to 30% PAN at about 75.degree. C.
Prior attempts to prepare dry blends of PAN powders with various organic solvents have resulted in relatively small amounts of solvents being incorporated. For instance, in U.S. Pat. No. 3,380,949, PAN powder was dry blended with DMF, alone or as a cosolvent with ethylene carbonate or DMSO, by slow addition of the solvent at temperatures up to 300.degree. F. However, large amounts of many solvents cannot be blended under such conditions according to prior techniques. DMF has been employed at concentrations up to 55 weight percent in certain circumstances. DMSO is especially difficult for dry blending with PAN because it has very high solvent power, even at ambient temperature. Attempts to prepare flowable PAN powders containing large amounts of dimethyl formamide by adding the DMF at low temperature have not been successful. It is an object of the present invention to prepare polyacrylonitrile powders in mixture with a suitable solvent for processing to form an extrusion mass of optimum concentration.